linux-kernel - Re: [PATCH v1 3/8] drm/etnaviv: Drop the second argument of the etnaviv_gem_new

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Message-ID: <e3a05204-61fe-2318-5f06-fd12addfe2e9@loongson.cn>
Date:   Tue, 18 Jul 2023 02:34:20 +0800
From:   suijingfeng <suijingfeng@...ngson.cn>
To:     Lucas Stach <l.stach@...gutronix.de>,
        Sui Jingfeng <sui.jingfeng@...ux.dev>,
        Russell King <linux+etnaviv@...linux.org.uk>,
        Christian Gmeiner <christian.gmeiner@...il.com>,
        David Airlie <airlied@...il.com>,
        Daniel Vetter <daniel@...ll.ch>
Cc:     loongson-kernel@...ts.loongnix.cn, etnaviv@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 3/8] drm/etnaviv: Drop the second argument of the
 etnaviv_gem_new_impl()

Hi,  Lucas


Thanks for you guidance!


On 2023/7/17 17:51, Lucas Stach wrote:
> Hi Jingfeng,
>
> Am Freitag, dem 23.06.2023 um 18:08 +0800 schrieb Sui Jingfeng:
>> From: Sui Jingfeng <suijingfeng@...ngson.cn>
>>
>> Because it is not used by the etnaviv_gem_new_impl() function,
>> no functional change.
>>
> I think it would make sense to move into the opposite direction: in
> both callsites of etnaviv_gem_new_impl we immediately call
> drm_gem_object_init with the size.

Really?

But there are multiple call path to the etnaviv_gem_new_impl() function.


Code path 1 (PRIME):

|- etnaviv_gem_prime_import_sg_table()

|--  etnaviv_gem_new_private()

|--- etnaviv_gem_new_impl(dev, size, flags, ops, &obj)

|--- drm_gem_private_object_init(dev, obj, size)


Code path 2 (USERPTR):

|- etnaviv_gem_new_userptr()

|--  etnaviv_gem_new_private()

|--- etnaviv_gem_new_impl(dev, size, flags, ops, &obj)

|--- drm_gem_private_object_init(dev, obj, size)


Code path 3 (construct a GEM buffer object for the user-space):

|- etnaviv_ioctl_gem_new()

|-- etnaviv_gem_new_handle()

|--- etnaviv_gem_new_impl(dev, size, flags, &etnaviv_gem_shmem_ops, &obj);

|---  drm_gem_object_init(dev, obj, size);


If I understand this correctly:


Code path 1 is for cross device (and cross driver) buffer-sharing,

Code path 2 is going to share the buffer the userspace,


*Only* the code path 3 is to construct a GEM buffer object for the 
user-space the userspace,

that is say, *only* the code path 3 need to do the backing memory 
allocation work for the userspace.

thus it need to call drm_gem_object_init() function, which really the 
shmem do the backing memory

allocation.


The code path 1 and the code path 2 do not need the kernel space 
allocate the backing memory.

Because they are going to share the buffer already allocated by others.

thus, code path 2 and code path 3 should call drm_gem_private_object_init(),

*not* the drm_gem_object_init() function.


When import buffer from the a specific KMS driver,

then etnaviv_gem_prime_import_sg_table() will be called.


I guess you means that drm_gem_private_object_init() (not the 
drm_gem_object_init() function)here ?


> A better cleanup would be to make
> use of the size parameter and move this object init call into
> etnaviv_gem_new_impl.

If I following you guidance, how do I differentiate the cases

when to call drm_gem_private_object_init() not drm_gem_object_init() ?

and when call drm_gem_object_init() not drm_gem_private_object_init()?


I don't think you are right here.

>
> Regards,
> Lucas
>
>> Signed-off-by: Sui Jingfeng <suijingfeng@...ngson.cn>
>> ---
>>   drivers/gpu/drm/etnaviv/etnaviv_gem.c | 7 +++----
>>   1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/etnaviv/etnaviv_gem.c b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
>> index b5f73502e3dd..be2f459c66b5 100644
>> --- a/drivers/gpu/drm/etnaviv/etnaviv_gem.c
>> +++ b/drivers/gpu/drm/etnaviv/etnaviv_gem.c
>> @@ -542,7 +542,7 @@ static const struct drm_gem_object_funcs etnaviv_gem_object_funcs = {
>>   	.vm_ops = &vm_ops,
>>   };
>>   
>> -static int etnaviv_gem_new_impl(struct drm_device *dev, u32 size, u32 flags,
>> +static int etnaviv_gem_new_impl(struct drm_device *dev, u32 flags,
>>   	const struct etnaviv_gem_ops *ops, struct drm_gem_object **obj)
>>   {
>>   	struct etnaviv_gem_object *etnaviv_obj;
>> @@ -591,8 +591,7 @@ int etnaviv_gem_new_handle(struct drm_device *dev, struct drm_file *file,
>>   
>>   	size = PAGE_ALIGN(size);
>>   
>> -	ret = etnaviv_gem_new_impl(dev, size, flags,
>> -				   &etnaviv_gem_shmem_ops, &obj);
>> +	ret = etnaviv_gem_new_impl(dev, flags, &etnaviv_gem_shmem_ops, &obj);
>>   	if (ret)
>>   		goto fail;
>>   
>> @@ -627,7 +626,7 @@ int etnaviv_gem_new_private(struct drm_device *dev, size_t size, u32 flags,
>>   	struct drm_gem_object *obj;
>>   	int ret;
>>   
>> -	ret = etnaviv_gem_new_impl(dev, size, flags, ops, &obj);
>> +	ret = etnaviv_gem_new_impl(dev, flags, ops, &obj);
>>   	if (ret)
>>   		return ret;
>>